Our invention relates generally to automotive transmissions of the type disclosed in U.S. Pat. Nos. 3,393,585; 3,613,484; 3,706,240; 3,714,836; U.S. Ser. No. 24,500 filed Mar. 11, 1987, now allowed, and U.S. Ser. No. 927,624 filed Nov. 6, 1984, now U.S. Pat. No. 4,744,269 applications and patents are assigned to the assignee of this invention.
Gear ratio change, both upshifts and downshifts, of an automatic transmission occur when various friction elements are engaged and disengaged selectively through operation of a hydraulic control. The time rate at which pressure increases in certain of the friction elements, engaged to produce a gear shift, is controlled through operation of an accumulator. Shift valves in the hydraulic circuit produce selectively a connection between hydraulic lines associated with the friction elements to be engaged and a source of line pressure.
A first control pressure has a magnitude that varies according to a required torque transmitting capacity of a friction element. A line modulator valve, continually supplied from the source of line pressure, produces a second control pressure having a magnitude that varies linearly with the magnitude of the first control pressure, but at a different rate than that of the first control pressure. The second control pressure produces zero pressure output over a low pressure range of the first control pressure.
An accumulator includes a cylinder having a piston movable within the cylinder defining a first volume connected to the second control pressure output by the line modulator valve, and a first volume communicating with line pressure selectively available to engage the friction element and associated with the respective friction element. An accumulator control valve directs line pressure immediately to the friction element during a brief period after line pressure is made available at the accumulator by the shift valve. The accumulator control valve also directs this pressure to the second volume of the accumulator through an orifice having a predetermined diameter to control the flow rate of fluid into the second volume of the accumulator.
Pressure in the friction element rises at a high rate during a first brief period before the second volume of the accumulator begins to expand against the effect of the second control pressure and springs, which force the accumulator piston to a position within the accumulator tending to maximize the first volume and minimize the second volume. As the second volume expands against this resistance, pressure in the friction element rises at a lower rate of increase than the rate of the first period, until the required torque capacity of the friction element is produced. When line pressure is removed from the accumulator control valve to disengage the friction element, the friction element, a clutch or brake, is vented through the shift valve that directs line pressure to the accumulator.